This invention relates to a process for treating the hydrophobic surface of a polymeric material, and particularly to such a process for making the surface wettable and hence receptive to electroless metal coatings.
Many polymeric materials such as the fluorocarbons have hydrophobic surfaces which do not form a strong bond with a metal coating or plating. Hence, the hydrophobic surface must be either mechanically or chemically treated to obtain better adhesion between the polymeric surface and the metal coating. An example of such material is Teflon.RTM., a DuPont trademark for tetrafluoroethylene resins. Teflon material is used because of its good electrical properties, chemical inertness, substantially zero moisture absorption, low coefficient of friction, and resistance to adhesion.
The prior art discloses many processes for treating hydrophobic surfaces of polymeric materials to make them adherent to metal coatings. One of those processes treats the polymeric surface with a sodium dispersion in napthalene. The sodium dispersion treatment is very expensive and has a very short useful life.
Other processes mechanically abrade the surface of the polymers, or etch the surfaces of the polymers with various acids.
One of the prior art processes relating to the treating of a polymeric surface to make it hydrophilic is taught in U.S. Pat. No. 3,471,313. That patent discloses a process for imparting a hydrophilic surface to polymers by treating the polymeric surface with an acid solution consisting of chromic acid and orthophosphoric acid. When a polymeric surface is so treated and then coated or plated with a metal, the result is an average bond strength between the metal plate and the surface of 41/2 pounds per inch of width. This strength is considerably less than highly unexpected bond or peel strength of 16-20 pounds per inch of width as will be discussed hereinafter. And even when orthophosphoric acid (H.sub.3 PO.sub.4) is boiled to produce polyphosphoric acid, the polyphosphoric acid does not make a polymeric surface appreciably more wettable.
U.S. Pat. No. 3,629,922 discloses that various plastics such as poly (haloethylenes) can be plated with metals by pretreatment with a compound, such as trihydroxymethyl phosphine.
U.S. Pat. No. 3,748,109 teaches the production of superconductors by applying, to a substrate of polytetrafluoroethylene, a low oxidation state phosphorous compound, applying a resist, removing the unprotected metal phosphorous compound, dissolving the resist, and subjecting the substrate to electroless or electrolytic treatment.
The use of phosphoric acid having a high P.sub.2 O.sub.5 content is known in electroless plating compositions as shown by Applicant's U.S. Pat. No. 3,689,292. The patent is directed to the treatment of a solder plate on a base metal to produce a lustrous adherent tin plate.
Applicant also has a pending U.S. patent application, Ser. No. 638,829, filed Dec. 8, 1975, now U.S. Pat. No. 4,014,715 relating to a solder cleaning and coating composition. The cleaning composition comprises a mineral acid, a phosphoric acid concentrate having a P.sub.2 O.sub.5 content of about 72-76%, formed by heating 54% phosphoric acid with a mono- or polysaccharide, a dibasic acid such as succinic acid, and thiourea. A wetting agent, such as Triton X-100 manufactured by Rohm and Haas, is also incorporated in the composition. Other similar wetting agents may be satisfactory.
To Applicant's knowledge, none of the prior art shows a process for treating a polymeric surface to make it wettable, that uses a modified pyro-phosphoric acid concentrate having a P.sub.2 O.sub.5 concentration of 70-82% (hereinafter referred to as MPA concentrate). Such MPA concentrates of Applicant have the formula R.sub.4 P.sub.2 O.sub.7 wherein R may be hydrogen.